To control heating and cooling costs, new buildings are being built in a more air-tight manner. One drawback of such construction is the decrease of fresh air flow rate into these buildings, and the resulting build-up of indoor air pollutants such as excess moisture, carbon dioxide, formaldehyde and various volatile organic compounds found in building materials, paints, furnishings, cleaning products and smoke. Opening a window to reduce this build-up results in a loss of heating or cooling energy from the home, negating the energy-saving effect of an air-tight construction.
HRVs are designed to provide proper ventilation to a well-insulated building while maintaining the temperature of the building by recovering the heating or cooling energy from the exhausted stale air.
An HRV is generally installed in a basement and is connected to air-supply and air-return vents through ductwork. An HRV has two air paths, a fresh air path through which fresh air enters the building and a stale air path through which stale air exits the building. Between the two air flow paths is the HRV core, which is an air-to-air heat exchanger. During the winter months, the cold fresh air entering the building is heated by the warm, stale air leaving the building, via the HRV core.
To ensure efficient operation of an HRV, the air flow rate through the HRV needs to be balanced. In other words, the rate at which fresh air enters a building and the rate at which stale air leaves the building needs to be made approximately equal. As a result, every HRV must be manually balanced upon installation. This is generally done by a qualified installation technician, and is accomplished by first measuring the mass air flow rate in each air path, and then adjusting one or more dampers in the air paths to balance the air flow. This process may be repeated by a person on a regular basis to ensure the continued efficiency of the HRV.
Several methods exist for measuring the air flow rate in each air path. A person may create a crude measuring device by taping the opening of a large plastic bag about an untwisted wire coat hanger. The person may then place the mouth of this bag about the stale air exhaust hood and count the number of seconds before the bag inflates. He or she may then place the mouth of the bag about the fresh air intake hood and do the same. By this process, the person may develop a crude estimate of the relative difference in mass air flow rate between the two air paths. Adjustment of the air flow rate in the air flow paths can then be made, normally by adjusting one or more dampers within the ductwork.
Another method of measuring the volume air flow rate in the air paths involves the drilling of a small hole in each air flow path and inserting a Pitot tube into each hole. As will be known by one skilled in the art, each Pitot tube will measure the total air pressure and the static air pressure at the point of insertion. The Pitot tubes are each connected to a separate manometer, which will effectively subtract the measured static pressure from the measured total pressure to obtain and display the velocity pressure at the point of insertion. (Total pressure equals static pressure plus velocity pressure). As velocity pressure is proportional to gas density and the square of the velocity volume air flow rate, one or more dampers can then be adjusted, if necessary, until the manometers readings are equal. The mass air flow is obtained by multiplying the average volumetric flow rate with the fluid density.
Yet another method of measuring the mass air flow rate in the air paths of an HRV with a view to balance the HRV is disclosed by U.S. Pat. No. 6,209,622, issued Apr. 3, 2001 to Lagace et al. The Lagace et al. method involves the determination of a static pressure difference between two points on each air flow path. The static pressure differences are then converted to air flow rate values by a technician using a conversion chart. One or more dampers can then be adjusted to balance the air flow rate in each air flow path.